Many operations carried out in refineries and petrochemical plants involve feeding a hydrocarbon/hydrogen stream to a reactor, withdrawing a reactor effluent stream of different hydrocarbon/hydrogen composition, separating the effluent into liquid and vapor portions, and recirculating part of the vapor stream to the reactor, so as to reuse unreacted hydrogen. Such loop operations are found, for example, in the hydroprocessor, catalytic reformer, and hydrogenation sections of most modern refineries, as well as in hydrogenation reactors in petrochemical plants.
The phase separation into liquid and vapor portions is often carried out in one or more steps by simply changing the pressure and/or temperature of the effluent. Therefore, in addition to hydrogen, the overhead vapor from the phase separation usually contains light hydrocarbons, particularly methane and ethane. In a closed recycle loop, these components build up, change the reactor equilibrium conditions and can lead to reduced product yield. This build-up of undesirable contaminants is usually controlled by purging a part of the vapor stream from the loop. Such a purge operation is unselective however, and, since the purge stream may contain as much as 80 vol % or more hydrogen, multiple volumes of hydrogen can be lost from the loop for every volume of contaminant that is purged. The purge stream may be treated by further separation in some downstream operation, or may simply pass to the plant fuel header. This purge stream is normally about 1-5% of the total hydrogen entering the reactor and may have a value of about $0.5-1.0 million/year of recoverable hydrogen values.
The impetus for hydrogen recovery in the reactor loop is two-fold. First, demand for hydrogen in refineries and petrochemical plants is high, and it is almost always more cost-effective to try to reuse as much gas as is practically possible than to meet the hydrogen demand entirely from fresh stocks. Secondly, it is desirable in most operations to maintain a high hydrogen partial pressure in the reactor. The availability of ample hydrogen during the reaction step prolongs the life of the catalyst, and suppresses the formation of non-preferred, low value products.
In refineries, separation of light hydrocarbons from hydrogen may be applied to effluent streams from hydrocrackers; hydrotreaters of various kinds, including hydrodesulfurization units; coking reactors; catalytic reformers; specific isomerization, alkylation and dealkylation units; steam reformers; and hydrogenation and dehydrogenation processes. Modern refineries improve gasoline yield by hydrogenating feedstocks containing unsaturated hydrocarbons, such as arise from cracking operations, to increase the saturated hydrocarbon content. For example, iso-octane is produced in this way for blending into the gasoline pool.
Petrochemicals that are produced by hydrogenation reactions include but are not limited to:
Cyclohexane, produced from benzene PA1 Aniline, produced from nitrobenzene PA1 Hexamethylenediamine, produced from adiponitrile PA1 Toluenediamine, produced from dinitrotoluene PA1 1,4 diacetoxybutane, produced from 1,4 diacetoxy-2-butene PA1 Benzene and naphthalene, produced from alkylbenzenes and alkylnaphthalenes PA1 (a) hydrogenating a hydrocarbon stream in a reactor; PA1 (b) subjecting an effluent stream comprising hydrogen and hydrocarbons from the hydrogenating step to at least one phase separation step, thereby producing a vapor stream comprising hydrogen and a light hydrocarbon; PA1 (c) performing a membrane separation step, comprising passing at least a portion of the vapor stream across the feed side of a polymeric membrane having a feed side and permeate side, and being selective for the light hydrocarbon over hydrogen; PA1 (d) withdrawing from the permeate side a permeate stream enriched in the light hydrocarbon compared with the vapor stream; PA1 (e) withdrawing from the feed side a residue stream enriched in hydrogen compared with the vapor stream; and, PA1 (f) recirculating at least a portion of the residue stream to the hydrogenating step.
Cyclohexane is produced by the hydrogenation of benzene in a single- or multi-step reaction in the presence of a nickel, platinum, or palladium catalyst. The reactor effluent is separated, and the crude liquid cyclohexane is stabilized to remove any remaining hydrogen. Separator and/or stabilizer off-gases may be recycled for reuse in the process.
Aniline is manufactured by circulating nitrobenzene and hydrogen in a reactor in the presence of a suitable metal catalyst. The reactor off-gases are filtered, cooled, and separated. Crude aniline is purified by vacuum distillation. Unreacted hydrogen is recycled, and catalyst is regenerated for reuse.
Hexamethylenediamine is produced by hydrogenating adiponitrile in the presence of ammonia and a suitable catalyst, typically Raney nickel. The crude product is separated and subjected to additional treatment. Unreacted hydrogen is recycled, and catalyst is regenerated for reuse.
Toluenediamine, an intermediate product in the manufacture of toluene diisocyanate, is manufactured in a multi-stage reaction of dinitrotoluene and hydrogen in the presence of a suitable catalyst. Excess hydrogen from the reactors is recycled.
1,4 Diacetoxybutane, an intermediate product in the manufacture of 1,4 butanediol, can be manufactured by the catalytic hydrogenation of 1,4 diacetoxy-2-butene. The reactor effluent is separated, typically in a two-stage flash process. The hydrogen off-gas is compressed and recycled to the reactor. The crude 1,4 diacetoxybutane is subjected to further processing steps to yield 1,4 butanediol.
Benzene and naphthalene are produced by the hydrodealkylation of alkylbenzenes and alkylnaphthalenes, respectively. The reactor effluent is separated and the liquid phase is sent to downstream treatment for product recovery. The hydrogen off-gas is recycled to the reactor.
Hydrogen recovery techniques that have been deployed in refineries include, besides simple phase separation of fluids, pressure swing adsorption (PSA) and membrane separation. U.S. Pat. No. 4,362,613, to Monsanto, describes a process for treating the vapor phase from a high-pressure separator in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the hydrocracker reactor. U.S. Pat. No. 4,367,135, also to Monsanto, describes a process in which effluent from a low-pressure separator is treated to recover hydrogen using the same type of hydrogen-selective membrane. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal.
U.S. Pat. No. 5,679,241, to ABB Lummus Global/Chemical Research and Licensing, describes a process for hydrogenation of certain acetylenes, dienes and olefins. The process includes a membrane separation step that uses a hydrogen-selective membrane to recover hydrogen from a light olefin/hydrogen stream. The hydrogen is then used to hydrogenate C.sub.5+ hydrocarbons for addition to the gasoline pool.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and "Optimizing Hydrocracker Hydrogen", by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063 and 4,836,833, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
U.S. Pat. No. 5,332,424, to Air Products, describes fractionation of a gas stream containing light hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,435,836; 5,447,559 and 5,507,856, which all relate to purification of streams from steam reformers. U.S. Pat. No. 5,634,354, to Air Products, discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen. U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
Hydrogenation processes for manufacture of chemical intermediates are well-documented in the patent literature. For example, U.S. Pat. No. 3,950,447, to Gryaznov, incorporated herein by reference in its entirety, describes a process that involves simultaneous dehydrogenation of one hydrocarbon and hydrogenation of a second hydrocarbon, yielding two new hydrocarbon products. U.S. Pat. No. 3,450,785, to UOP, incorporated herein by reference in its entirety, describes catalytic hydrogenation of benzene to cyclohexane, using recycled hydrogen that has been purified by steam reforming. U.S. Pat. No. 3,592,864, also to UOP, incorporated herein by reference in its entirety, describes a cyclohexane manufacturing process that does not use any recycled hydrogen. U.S. Pat. No. 3,461,182, to Phillips Petroleum, incorporated herein by reference in its entirety, describes a process for hydrogenating benzene to produce cyclohexane in the presence of methylcyclopentane, which forms an azeotrope with any unreacted benzene, yielding a higher-purity cyclohexane product. U.S. Pat. No. 5,856,603, to Engelhard Corp., incorporated herein by reference in its entirety, describes a benzene hydrogenation process using an improved catalyst to enhance cyclohexane yield and minimize formation of by-products.
U.S. Pat. No. 4,265,834, to Bayer Akteingesellschaft, incorporated herein by reference in its entirety, describes an improved catalyst for use in a hydrogenation process to convert nitrobenzene to aniline. U.S. Pat. No. 4,415,754, to DuPont, incorporated herein by reference in its entirety, describes an aniline manufacture process with an improved method for removing impurities. U.S. Pat. No. 3,758,584, also to DuPont, incorporated herein by reference in its entirety, describes an improved hydrogenation catalyst for hexamethylenediamine production. U.S. Pat. No. 3,821,305, to Montedison Fibre, incorporated herein by reference in its entirety, describes a hydrogenation process for improved yield and quality of the hexamethylenediamine product. U.S. Pat. No. 3,935,264, to Olin Corp., incorporated herein by reference in its entirety, describes an improved hydrogenation process that suppresses formation of undesirable by-products in the production of toluenediamine from dinitrotoluene. U.S. Pat. No. 4,224,249, to Air Products, incorporated herein by reference in its entirety, describes the production of toluenediamine by an improved hydrogenation process that minimizes degradation of catalyst.